Data storage systems including data storage media such as disc drives are commonly used in a wide variety of devices to store large amounts of data in a form that can be made readily available to a user. While commonly used in computing devices such as personal computers, workstations, and laptops, disc drives have also been incorporated into personal music devices and in other applications.
In general, a disc drive includes one or more storage discs that are rotated by a spindle motor. The surface of each of the one or more storage discs is divided into a series of data tracks. The data tracks are spaced radially from one another across a band having an inner diameter and an outer diameter. The data tracks extend generally circumferentially around the disc and can store data in the form of magnetic transitions within the radial extent of a given track. An interactive element, such as a magnetic transducer, is used to sense the magnetic transitions to read data from the given track. In addition, the interactive element can transmit an electric signal that causes a magnetic transition on the disc surface to write data to the given track.
The interactive element is mounted to an arm of an actuator. The interactive element is then selectively positioned by the actuator arm over a given data track of the disc to either read data from or write data to the given data track of the disc, as the disc rotates adjacent the transducer. The actuator arm is, in turn, mounted to a voice coil motor that can be controlled to move the actuator arm relative to the disc surface.
An embedded servo control system is typically used to control the position of the actuator arm to ensure that the interactive element is properly centered over the given data track during either a read or write operation. In the embedded servo control system, servo position information can be recorded on the disc surface between written data blocks, and periodically read by the interactive element for use in a closed loop control of the voice coil motor to position the actuator arm. Alternatively, dedicated servo tracks or surfaces can be used.
In modern disc drive architectures utilizing embedded servo control systems, each data track is divided into a number of data sectors for storing fixed size data blocks, one per sector. Associated with the data sectors are a series of servo sectors, generally equally spaced around the circumference of the data track. The servo sectors can be arranged between data sectors or arranged independently of the data sectors such that the servo sectors split data fields of the data sectors. The servo sectors can be read to determine the position of the interactive element relative to the disc drive surface for the purposes of reading data from the given data track.
The process of moving the actuator arm from one position to another is divided into a number of stages, including a seek stage, a settle stage, and a track following stage. The settle stage can be further divided into an early settle stage and a late settle stage. Each stage has different performance requirements and thus requires different capabilities from the embedded servo control system. Examples of the performance requirements that must be met by the overall design include reducing seek time and post seek oscillation, reducing the effects of operational vibration, reducing steady state tracking error, improving acoustics, and reducing power consumption and processing overhead. In addition, smooth transitions between the different stages of the embedded servo control system will ensure adequate performance m high performance disc drives by reducing transient signals that may occur during transitions. For example, any transients generated during a transition between one stage and another stage can degrade the performance of the disc drive. In addition, controller designs that incorporate the same architecture from one stage to the next can impose constraints on the performance of the controller architecture from one stage to another.